LOCKF(3C)LOCKF(3C)NAMElockf - record locking on files
C SYNOPSIS
#include <unistd.h>
int lockf (int fildes, int function, off_t size);
DESCRIPTION
The lockf command will allow sections of a file to be locked; advisory or
mandatory write locks depending on the mode bits of the file [see
chmod(2)]. Locking calls from other processes which attempt to lock the
locked file section will either return an error value or be put to sleep
until the resource becomes unlocked. All the locks for a process are
removed when the process terminates. [See fcntl(2) for more information
about record locking.]
Fildes is an open file descriptor. The file descriptor must have
O_WRONLY or O_RDWR permission in order to establish lock with this
function call.
Function is a control value which specifies the action to be taken. The
permissible values for function are defined in <unistd.h> as follows:
#define F_ULOCK 0 /* Unlock a previously locked section */
#define F_LOCK 1 /* Lock a section for exclusive use */
#define F_TLOCK 2 /* Test and lock a section for exclusive use */
#define F_TEST 3 /* Test section for other processes locks */
All other values of function are reserved for future extensions and will
result in an error return if not implemented.
F_TEST is used to detect if a lock by another process is present on the
specified section. F_LOCK and F_TLOCK both lock a section of a file if
the section is available. F_ULOCK removes locks from a section of the
file.
Size is the number of contiguous bytes to be locked or unlocked. The
resource to be locked starts at the current offset in the file and
extends forward for a positive size and backward for a negative size (the
preceding bytes up to but not including the current offset). If size is
zero, the section from the current offset through the largest file offset
is locked (i.e., from the current offset through the present or any
future end-of-file). An area need not be allocated to the file in order
to be locked as such locks may exist past the end-of-file.
The sections locked with F_LOCK or F_TLOCK may, in whole or in part,
contain or be contained by a previously locked section for the same
process. When this occurs, or if adjacent sections occur, the sections
are combined into a single section. If the request requires that a new
element be added to the table of active locks and this table is already
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LOCKF(3C)LOCKF(3C)
full, an error is returned, and the new section is not locked.
F_LOCK and F_TLOCK requests differ only by the action taken if the
resource is not available. F_LOCK will cause the calling process to
sleep until the resource is available. F_TLOCK will cause the function
to return a -1 and set errno to [EACCES] error if the section is already
locked by another process.
F_ULOCK requests may, in whole or in part, release one or more locked
sections controlled by the process. When sections are not fully
released, the remaining sections are still locked by the process.
Releasing the center section of a locked section requires an additional
element in the table of active locks. If this table is full, an
[EDEADLK] error is returned and the requested section is not released.
A potential for deadlock occurs if a process controlling a locked
resource is put to sleep by accessing another process's locked resource.
Thus calls to lockf or fcntl scan for a deadlock prior to sleeping on a
locked resource. An error return is made if sleeping on the locked
resource would cause a deadlock.
Sleeping on a resource is interrupted with any signal. The alarm(2)
command may be used to provide a timeout facility in applications which
require this facility.
The lockf utility will fail if one or more of the following are true:
[EBADF]
Fildes is not a valid open file.
[EACCES]
Cmd is F_TLOCK or F_TEST and the section is already locked by
another process.
[EDEADLK]
Cmd is F_LOCK and a deadlock would occur. Also the cmd is either
F_LOCK, F_TLOCK, or F_ULOCK and the number of entries in the lock
table would exceed the number allocated on the system.
NOTES
Locks are on files, not file descriptors. That is, file descriptors
duplicated through dup(3C) do not result in multiple instances of locks,
but rather multiple references to the same locks. Thus if any of the
descriptors associated with the same file are closed, the locks
associated with the file are lost.
SEE ALSOchmod(2), close(2), creat(2), fcntl(2), intro(2), open(2), read(2),
write(2).
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LOCKF(3C)LOCKF(3C)DIAGNOSTICS
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
WARNINGS
Unexpected results may occur in processes that do buffering in the user
address space. The process may later read/write data which is/was
locked. The standard I/O package is the most common source of unexpected
buffering.
Because in the future the variable errno will be set to EAGAIN rather
than EACCES when a section of a file is already locked by another
process, portable application programs should expect and test for either
value.
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